The present invention relates to liquid crystal carboxylic acid esters and their use for modifying the dielectric anisotropy (DCA) of liquid crystal dielectrics.
Liquid crystal dielectrics with a distinctly negative DCA are required for electrooptical display elements, the mode of action of which is based on the phenomenon of dynamic scattering or the deformation of aligned phases (DAP effect). The threshold voltage for the operation of such display elements becomes lower the more negative the DCA of the liquid crystal dielectric is. Another fundamental requirement for such dielectrics is that the nematic phase have a broad temperature range, which includes room temperature.
Among the liquid crystal base materials customarily used in practice for such dielectrics, which at the same time also possess the requisite chemical stability and as a rule are mixtures of two or more compounds from the classes of the azoxybenzenes, the phenyl benzoates, the phenyl thiobenzoates and the phenylcyclohexanes, there is none which has a markedly negative DCA. In order to produce liquid crystal dielectrics with a markedly negative DCA from such mixtures, liquid crystal compounds possessing a very highly negative DCA are added to these mixtures; preferably, the 4,4"-disubstituted 2"-cyanophenyl benzoyloxybenzoates according to German Pat. No. 2,240,864 or the p,p"-disubstituted 2"-cyanophenyl biphenylylcarboxylates according to German Offenlegungsschrift No. 2,613,293 (U.S. Pat. No. 4,073,742) are used as such liquid crystal compounds having a very highly negative DCA. However, it has been found that these compounds which lower the DCA either are not sufficiently soluble in the liquid crystal base materials to obtain a negative DCA of the desired magnitude or effect an undesired shift in the temperature range of the nematic phase. It is true that it is possible to a limited extent to circumvent the solubility problems by the use of several different DCA-modifying compounds and also to widen or correct the temperature range of the nematic phase, if this has been narrowed or shifted in an undesirable manner, by the addition of p,p"-disubstituted phenyl benzoyloxybenzoates according to German Offenlegungsschrift No. 2,139,628 (U.S. Pat. No. 4,002,670) or of p,p"-disubstituted phenyl biphenylylcarboxylates according to German Offenlegungsschrift No. 2,450,088 (U.S. Pat. No. 4,065,489). In all cases the liquid crystal dielectrics thus obtained are, however, multi-component mixtures, the production of which on a relatively large scale is very involved.